Re: [PATCH] /proc/PID/smaps: Add PMD migration entry parsing

[Huang\, Ying]

Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx> writes:

> On 31/03/2020 11.56, Huang, Ying wrote:
>> From: Huang Ying <ying.huang@xxxxxxxxx>
>>
>> Now, when read /proc/PID/smaps, the PMD migration entry in page table is simply
>> ignored.  To improve the accuracy of /proc/PID/smaps, its parsing and processing
>> is added.
>>
>> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
>> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
>> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
>> Cc: Zi Yan <ziy@xxxxxxxxxx>
>> Cc: Vlastimil Babka <vbabka@xxxxxxx>
>> Cc: Alexey Dobriyan <adobriyan@xxxxxxxxx>
>> Cc: Michal Hocko <mhocko@xxxxxxxx>
>> Cc: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
>> Cc: "JÃrÃme Glisse" <jglisse@xxxxxxxxxx>
>> Cc: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
>> ---
>>   fs/proc/task_mmu.c | 16 ++++++++++++----
>>   1 file changed, 12 insertions(+), 4 deletions(-)
>>
>> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
>> index 8d382d4ec067..b5b3aef8cb3b 100644
>> --- a/fs/proc/task_mmu.c
>> +++ b/fs/proc/task_mmu.c
>> @@ -548,8 +548,17 @@ static void smaps_pmd_entry(pmd_t *pmd, unsigned long addr,
>>   	bool locked = !!(vma->vm_flags & VM_LOCKED);
>>   	struct page *page;
>
>         struct page *page = NULL;

Looks good.  Will do this in the next version.

>>   -	/* FOLL_DUMP will return -EFAULT on huge zero page */
>> -	page = follow_trans_huge_pmd(vma, addr, pmd, FOLL_DUMP);
>> +	if (pmd_present(*pmd)) {
>> +		/* FOLL_DUMP will return -EFAULT on huge zero page */
>> +		page = follow_trans_huge_pmd(vma, addr, pmd, FOLL_DUMP);
>> +	} else if (unlikely(is_swap_pmd(*pmd))) {
>> +		swp_entry_t entry = pmd_to_swp_entry(*pmd);
>> +
>> +		VM_BUG_ON(!is_migration_entry(entry));
>> +		page = migration_entry_to_page(entry);
>
>                 if (is_migration_entry(entry))
>                         page = migration_entry_to_page(entry);
>
> Seems safer and doesn't add much code.

With this, we lose an opportunity to capture some bugs during debugging.
Right?

Best Regards,
Huang, Ying

>> +	} else {
>> +		return;
>> +	}
>>   	if (IS_ERR_OR_NULL(page))
>>   		return;
>>   	if (PageAnon(page))
>> @@ -578,8 +587,7 @@ static int smaps_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
>>     	ptl = pmd_trans_huge_lock(pmd, vma);
>>   	if (ptl) {
>> -		if (pmd_present(*pmd))
>> -			smaps_pmd_entry(pmd, addr, walk);
>> +		smaps_pmd_entry(pmd, addr, walk);
>>   		spin_unlock(ptl);
>>   		goto out;
>>   	}
>>